Does all your user will be notified that you will monitoring the Instant Messages conversation?

You know Jahanzeb writing a solution is 2 mins Job but as a security consultant & Licenced Penetration tester my job is to make sure I am NOT knowingly encouraging to violates people privacy.

I am writing a solution here which is not that Interactive but will solve the purpose.

You can do the tcp dump in your FC machine of your internet traffic of specific IP and/or Port and make a capture file, then you can use software like Shadow IM Sniffer to import the capture file and it will re-generate the IM traffic so it can be readable.

Let me again tell you there are more robust / automated / solutions that I can proposed here BUT I think this will solve the purpose AND / OR I am not 100% sure that this is really for SECURITY REASON.